If he can reuse the tower, harddisk, Windows OS, (and SSD), this would be a viable build:
PCPartPicker part list:
http://uk.pcpartpicker.com/list/H6Cqm8
Price breakdown by merchant: http://uk.pcpartpicker.com/list/H6Cqm8/by_merchant/
CPU: Intel Core i5-6500 3.2GHz Quad-Core Processor (£181.90 @ Amazon UK)
Motherboard: Gigabyte GA-H110M-H Micro ATX LGA1151 Motherboard (£48.99 @ Amazon UK)
Memory: G.Skill Ripjaws V Series 8GB (2 x 4GB) DDR4-2400 Memory (£32.87 @ CCL Computers)
Video Card: Sapphire Radeon RX 480 4GB NITRO+ 4G Video Card (£198.99 @ Ebuyer)
Power Supply: EVGA 600B 600W 80+ Bronze Certified ATX Power Supply (£54.22 @ CCL Computers)
Total: £516.97
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-08-30 19:47 BST+0100
Otherwise, with £500, he would have to go with i3-6100... which I leave this to others to comment because the value-for-quality will start to drop.